From the modest feeding regiment the OP describes (and from the smallish size of the fish of under 3' at 8 years), I think it has nothing to do with the overeating.
1. OP discounts the possibility of the RTC swallowing a foreign/inedible object. Are you absolutely sure? RTC are known to swallow heater guards, thermometers, decorations, etc. I'd massage his tummy a bit to see if I feel anything hard and shaped.
2. From reading all the info on this thread, I'd blame bread as the culprit. But having never fed my RTCs bread and never reading about any one else doing it, I can't be very sure. In any case, if the fish survives, please stop with the bread. RTCs are predators. They do take a fallen fruit in the wild too.
3. RTCs do get bloated sometimes and then it passes. Usually, keepers never know why.
4. It may have contracted a parasite, like an intestinal worm. Metronidazole, AFAIK, is largely an antiparasitic with some secondary antibiotic properties. But against worms, I'd recommend praziquantel - great stuff and safe too. Given often to cats and dogs too.
5. In one trustworthy study, high nitrates have been linked to a lazy digestion, bloating, and swimming bladder problems (float - just like your fish appears to suffer from) in goldfish.
6. Many RTCs have dark skin on their tummies. I've never heard of the RTC tummy color being linked with overeating. Yet, indeed, if the tummy does change color, it is a bad sign that the fish is experiencing GI problems.
Hope this helps. These are just my opinions. Good luck and keep us posted. I liked the suggestions of the laxative and anti-parasitic meds delivered by syringe directly into his tummy.
